A known method of contactless crack measurements of a metallic material is to utilise optical means. The metallic material may be irradiated by light wherein a crack may be detected by means of an optical sensor such as a camera. Drawbacks with optical methods are that it is not possible to detect cracks which are not visible on the surface of the metallic material, and that colour variations in the metallic material may be interpreted as cracks by the optical sensor. Optical methods have been proved to be difficult to use in other applications than for inspection of completely clean and smooth metal surfaces.
Inspection of metallic materials in for instance steel production has been made utilising inductive techniques. When using an inductive technique a current is induced in the metallic material, e.g. a slab or a metal sheet, by means of a time-varying magnetic field generated by a transmitter coil fed with a likewise time-varying current. When the induced current encounters a crack in the metallic material, the crack constitutes an obstacle to the induced current. As a result, the crack alters the induced current at the crack as compared to a metallic material without a crack. The altered current provides a change in the magnetic field around the current. The change in the magnetic field is measured by a receiver coil, whereby it can be determined that a crack is present in the inspected surface portion of the metallic material.
There are several drawbacks with the induction techniques used today for crack detection in metallic materials. Several parameters other than a crack depth may for example influence changes in the magnetic field. Examples of such parameters are the distance between the coils and the object to be measured, magnetic oxide on the surface of the object, changes in the physical characteristics in the material of which the object is made, the position of the crack in relation to the coils, and the length of the crack. To this end, when a change is measured by a receiver coil, it may be difficult to determine whether this change is due to the crack depth or another parameter which may vary. Due to the fact that it is difficult to keep influencing parameters constant, it has been difficult to use inductive techniques for crack inspection of irregular surfaces such as casted metallic surfaces.